As is known, numerical switching telephone exchanges, or private branch exchanges commonly referred to as PABXs (Private Automatic Branch Exchanges) of the digital type, incorporate interfacing integrated electronic circuits intended for driving voice transmissions over a plurality of telephone subscriber lines.
Such interface circuits, which may be of a commercial type known as SLIC (Subscriber Line Interface Circuit), are connected between the telephone subscriber lines and the remaining circuitry internal of the telephone exchange, and operative to supply on the telephone line a given line voltage and current to suit the line resistive load.
Such devices, as well as the subscriber line itself, require periodical testing for functionality, which tests show to be most effective when carried out during routine operation of the system.
To enable such testing operations, the prior art has proposed some relays for connection in the line, using complicated sectioning and switching structures, so as to run the SLIC and subscriber line to a centralized apparatus adapted to take the various measurements on a remote basis.
The tests are normally carried out during night hours to make best use of the less dense line traffic.
This prior art approach, while achieving its objective and being at present the only one which is applied irrespective of the system electromechanical, hybrid, or fully electronic type, still has the disadvantage of requiring a large number of relays and switch contacts. These components are recognized to be cost-intensive and low in reliability.
Another disadvantage of that prior approach is that the remote central apparatus for taking the measurements cannot help being somewhat complicated circuit-wise if it is to analyze all of the signals that propagate from the sectioning relays over so-called transfer buses.